Manufacturing method of a lead frame

ABSTRACT

In a semiconductor device, a lead frame made of a copper alloy prevents exfoliation occurring near the surface of the lead frame. A copper oxide layer is formed on the base material made of a copper alloy by immersing the base material into a solution of a strong oxidizer. The copper oxide layer serves as an outermost layer and consists of a copper oxide other than a copper oxide in the form of needle crystals.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. Ser. No.10/336,716, filed Jan. 6, 2003, which is based upon and claims thebenefit of priority from the prior Japanese Patent Application No.2002-166898, filed Jun. 7, 2002, the entire contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to semiconductor devices and,more particularly, to a semiconductor device which is formed byencapsulating a semiconductor element mounted on a lead frame by a sealresin.

2. Description of the Related Art

FIG. 1 is a cross-sectional view of a semiconductor device using aconventional lead frame. The semiconductor device 1 shown in FIG. 1 hasa so-called lead type package, which is provided with many leadterminals extending from a resin seal part which encapsulates asemiconductor chip.

In FIG. 1, a semiconductor chip 2 is fixed on a die stage 4 of a leadframe 3 by a die-bonding material 5. Electrodes of the semiconductorchip 2 are connected to inner leads 7 by bonding wires 6, and thesemiconductor chip 2, the die stage 4, the bonding wires 6 and the innerleads 7 are encapsulated by a seal resin 8. From the seal resin 8, outerleads 9 extend as terminals for external connection.

Generally, a lead frame is formed by processing and patterning a copperalloy plate by stamping, etching, etc. After patterning the copper alloyplate, silver (Ag) plating is applied at the end of each inner lead soas to facilitate wire bonding. If necessary, an organic discolorationpreventing agent is applied to the whole lead frame. Zinc (Zn), lead(Pb), chromium (Cr), etc. are contained as additives in the copper (Cu)alloy forming the lead frame.

In a manufacturing process of the above-mentioned semiconductor device1, the surface of the lead frame 3 is oxidized thermally in a heatingprocess such as a wire-bonding process. That is, heating of the leadframe 3 forms a thin film of copper oxide on the surface. In such acase, a phenomenon occurs in which a very small amount of additiveelements in the copper alloy is separated and condensed near a boundarybetween a copper oxide layer and a base material. Such a part into whichthe additive elements are condensed has a comparatively brittlecharacteristic. That is, thermal oxidation of the base material of thecopper alloy forms a brittle layer between the copper oxide layer on thesurface and the inner base material.

Additionally, the discoloration-preventing agent applied to the surfaceof the lead frame 3 also forms a brittle layer when a thermal oxidationfilm is formed in a heating process such as a wire-bonding process. Thatis, a part of elements contained in the discoloration preventing agentmay form a brittle layer near the boundary between the copper base andthe copper oxide film.

If a semiconductor device is heated in a solder reflow process in a casein which the semiconductor device, which is formed by encapsulating thelead frame 3 in which the above-mentioned brittle layer is formed, ismounted onto a mounting substrate, etc., a crack may occur in thebrittle layer. If a moisture in the seal resin turns into steam andenters such a crack, as shown in FIG. 2, exfoliation may occur betweenthe lead frame 3 (a copper alloy which is the material of the die stage4) and the seal resin 8 (copper oxide layer of the surface of the diestage 4), and there may occur a problem of package cracking or internalcracking. Such a problem tends to appear more notably, if a mountingtemperature rises due to use of a lead-free solder.

Additionally, an effect similar to the effect of the present inventionmay be obtained in a blackening treatment which forms needle crystals ofcupric oxide (CuO) on a surface of a copper alloy process. However,depending on the state of formation of copper dioxide, exfoliation mayoccur between the copper oxide and the base material of the copper alloywhen the completed semiconductor device is subjected to a solder reflowprocess, and there is no way to distinguish them by their appearance.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide an improvedand useful semiconductor device in which the above-mentioned problemsare eliminated.

A more specific object of the present invention is to provide a leadframe made of copper alloy which can prevent exfoliation occurring nearthe surface of the lead frame and a semiconductor device using such alead frame.

In order to achieve the above-mentioned objects, there is providedaccording to one aspect of the present invention a lead framecomprising: a base material made of a copper alloy; and a copper oxidelayer formed by contacting the lead frame with a solution of a strongoxidizer, the copper oxide layer serving as an outermost layer andconsisting of a copper oxide other than a copper oxide in the form ofneedle crystals.

According to the above-mentioned invention, since the copper oxide layeris previously formed on the base material of the lead frame, the basematerial of the lead frame is not oxidized thermally and there is nobrittle layer formed in the base material in a manufacturing process ofa semiconductor device. Thus, even if the semiconductor device is heatedafter a resin encapsulation, the package of the semiconductor device isprevented from being swollen or cracked. Additionally, the copper oxidelayer can be formed by merely immersing the lead frame into the solutionof the strong oxidizer, which reduces an increase in the manufacturingcost of the semiconductor device.

Additionally, in the lead frame according to the present invention, athickness of the copper oxide layer is preferably 10 to 1000 Angstroms.Since the copper oxide layer is very thin, the copper oxide layer doesnot change into a needle crystal layer. Thus, the copper oxide layer canbe formed as a stable single layer on the surface of the base material.

Additionally, there is provided according to another aspect of thepresent invention a manufacturing method of a lead frame, comprising:configuring a base material made of a copper alloy into a predeterminedpattern; applying plating to a part of the base material; and forming acopper oxide layer as an outermost layer on a surface of the basematerial by immersing the base material into a solution of a strongoxidizer, the copper oxide layer consisting of a copper oxide other thana copper oxide in the form of needle crystals.

In the manufacturing method according to the present invention, a timeperiod for immersing the base material into the solution of the strongoxidizer may be adjusted so as to take the base material out of thesolution of the strong oxidizer before the copper oxide changes to aneedle crystal state.

Additionally, there is provided according to another aspect of thepresent invention a semiconductor device comprising: a lead frame havinga copper oxide layer formed on a base material made of a copper alloy,the copper oxide layer formed by contacting the lead frame with asolution of a strong oxidizer and serving as an outermost layer, thecopper oxide layer consisting of a copper oxide other than a copperoxide in the form of needle crystals; a semiconductor element mounted ona predetermined portion of the lead frame; and a seal resinencapsulating the semiconductor device.

In the semiconductor device according to the present invention, athickness of the copper oxide layer may be 10 to 1000 Angstroms.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor device using aconventional lead frame;

FIG. 2 is a cross-sectional view of the semiconductor device shown inFIG. 1, which shows exfoliation occurring between a die stage and a sealresin;

FIG. 3 is a plan view of a lead frame used in a semiconductor deviceaccording to an embodiment of the present invention;

FIG. 4 is an enlarged cross-sectional view of a die stage of the leadframe shown in FIG. 3;

FIG. 5 is a plan view of the lead frame showing a state wherewire-bonding is performed after mounting a semiconductor chip 2 on a diestage;

FIG. 6 is an enlarged side view showing the semiconductor chip mountedon the die stage; and

FIGS. 7A and 7B are illustrations for explaining a processes of forminga copper oxide layer according to the present invention in comparisonwith a process of a conventional blackening treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to the drawings, ofembodiments of the present invention.

FIG. 3 is a plan view of a lead frame 10 used in a semiconductor deviceaccording to an embodiment of the present invention. FIG. 4 is anenlarged cross-sectional view of a die stage 11 of the lead frame 10shown in FIG. 3. The lead frame 10 used in the semiconductor deviceaccording to the present invention is formed, similar to theconventional lead frame, by processing and patterning a copper alloyplate as a base. Generally, the copper alloy for lead frames contains avery small amount of zinc (Zn), lead (Pb), chromium (Cr), etc. asadditive elements. The patterning of the copper alloy plate is performedby a known technique such as stamping and etching, as usual. Afterpatterning the copper alloy plate so as to form the configuration of thelead frame 10, silver (Ag) plating is applied to the end of each ofinner leads 12. Thus far, the same process is performed as theconventional lead frame.

Although an organic discoloration preventing agent may be applied afterthe silver plating in the conventional lead frame, the lead frame 10according to the present invention omits the discoloration preventingagent on the surface, and a thin layer of copper oxide is formed on thesurface of the lead frame 10. That is, the conventional lead frame iscompleted after applying a discoloration preventing agent onto thesurface of the copper alloy plate, while in the lead frame 10 accordingto the present invention, the copper alloy, which is a base material, isoxidized by a special method mentioned later so as to form a copperoxide layer 14 as an outermost layer on the surface of the lead frame 10(refer to FIG. 4).

As mentioned above, the lead frame 10 is completed after the copperoxide layer 14 is formed on the surface of the lead frame 10, especiallythe surface of a die stage 11. Then, the lead frame 10 is used formanufacture of a semiconductor device.

Fundamental composition of the semiconductor device according to theembodiment of the present invention is the same as the semiconductordevice shown in FIG. 1 except for the copper oxide layer 14 being formedin the lead frame 10. FIG. 5 is a plan view of the lead frame 10 showinga state where wire-bonding is performed after mounting the semiconductorchip 2 on the die stage 11. FIG. 6 is an enlarged side view showing thesemiconductor chip 2 mounted on the die stage 11.

In the production process of the semiconductor device, the semiconductorchip 2 is first mounted onto the die stage 11 of the lead frame 10 witha die-bonding material 5 provided therebetween. Then, electrodes of thesemiconductor chip 2 and the silver-plated parts of the inner leads 12are connected by bonding wires 6. Thereafter, the die stage 11, thesemiconductor chip 2, the bonding wires 6 and the inner leads 12 areencapsulated by a seal resin 8.

The semiconductor device according to the present embodiment uses thelead frame 10 in which the copper oxide layer 14 is formed on thesurface of the base material. For this reason, even if the lead frame 10is heated in a wire-bonding process, the copper in the base material ofthe lead frame 10 is not oxidized thermally. Therefore, there is nobrittle layer, which may be formed between the copper alloy as the basematerial and the copper oxide layer due to condensation of the additiveelements during the thermal oxidation process, thereby preventingpackage selling or cracking caused by the brittle layer.

A description will now be given, with reference to FIGS. 7A and 7B, of aformation process of the copper oxide layer 14 according to the presentembodiment. FIGS. 7A and 7B are illustrations for explaining a processof forming the copper oxide layer 14 in comparison with a case where thecopper oxide layer 14 is not formed. FIG. 7A shows the change of statenear, the surface of the lead frame in the case where the copper oxidelayer is not formed, and FIG. 7B shows the change of state near thesurface of the lead frame in the case where the copper oxide layer 14according to the present embodiment is formed.

First, a base material 21, which constitutes a copper alloy plate, ispatternized so as to from the plate into the configuration of the leadframe. In this state, as shown in FIG. 7A-(a) and FIG. 7B-(a), thecopper alloy of the base material 21 is exposed on the surface of thelead frame.

Next, an oxidation treatment is applied to the lead frame 10 accordingto the present embodiment as shown in FIG. 7B-(b). This treatment is notapplied in the case of FIG. 7A where the copper oxide layer is notformed. The oxidation treatment is performed by immersing the lead frame10 into a solution of a strong oxidizer. Consequently, the copper in thebase material 21 is oxidized by the strong oxidizer, and the copperoxide layer 14 is formed. Although the copper oxide layer 14 mainlyconsists of cuprous oxide(Cu₂O), the copper oxide layer 14 also containscupric oxide (CuO).

In the oxidization by the solution of a strong oxidizer, there is noseparation of additive elements between the copper oxide layer 14 andthe copper alloy of the base 21, and, thus, the brittle layer due tocondensation of the additive elements is not formed. Additionally, evenif the discoloration preventing agent is applied on the surface of thebase material, a component of the discoloration preventing agent is notcontained in the base material since the component is dissolved into thesolution of the strong oxidizer, and, thus, a brittle layer is notformed.

Here, there is a so-called blackening treatment as a process for forminga copper oxide layer by immersing a copper alloy into a solution of astrong oxidizer. The blackening treatment is a process for forming aneedle crystal layer of cupric oxide (CuO) on the surface of a copperalloy, and is called as a blackening treatment since the color of theneedle crystal layer of cupric oxide (CuO) is black. Generally, theblackening treatment is a process to improve adhesion between a sealresin and a lead frame by configuring the surface of the lead frame intoneedle-like shape.

The solution of the strong oxidizer used for the blackening treatmentis, for example, a mixed solution of sodium chlorite, sodium hydroxideand potassium peroxydisulfate. The needle crystal layer of cupric oxide(CuO) is formed by immersing the copper alloy into such a mixed solutionfor 3-10 minutes at a temperature of around 100° C.

Although the copper oxide layer 14 formed on the base material 21 in thepresent embodiment can also be formed using the mixed solution of thestrong oxidizer for the above-mentioned blackening treatment, the copperoxide layer 14 is not a needle crystal layer. Namely, in theconventional blackening treatment, chemical reactions are continueduntil a copper oxide layer of the surface becomes a needle crystal layerof cupric oxide (CuO). On the other hand, the copper oxide layer 14according to the present embodiment mainly consists of cuprouse oxide(Cu₂O), which is formed by taking the lead frame out of the mixedsolution of a strong oxidizer prior to the formation of cupric oxide(CuO) which turns to the needle crystal layer.

Therefore, the time of oxidation treatment according to the presentembodiment must be remarkably shorter than the time required by theconventional blackening treatment. Additionally, although the outermostlayer of the lead frame, which has been subjected to the blackeningtreatment, is the needle crystal layer of cupric oxide (CuO), the leadframe 10 according to the present embodiment has the copper oxide layer14 as the outermost layer which is not the needle crystal layer.Further, the thickness of the copper oxide layer 14 according to thepresent embodiment is remarkably smaller than the thickness of theneedle crystal layer formed by the blackening treatment, and issufficient in the order of about 10 to 1000 Å.

As mentioned above, since the copper oxide layer 14 according to thepresent embodiment can be formed by merely immersing the lead frame intoa solution of a strong oxidizer for a very short tine, the copper oxidelayer 14 can be easily formed without increasing the manufacturing costof the lead frame. Moreover, the copper oxide layer 14 can be very thin,and can be formed as a stable cuprous oxide (Cu₂O) layer.

Next, when the semiconductor device is formed using the lead frame, thelead frame is heated in a wire-bonding process. At this time, as shownin FIG. 7A-(c), a copper oxide layer 22 is formed as shown in FIG.7A-(c) due to thermal oxidation of copper of the exposed base material21 in the case where the copper oxide layer 14 is not formed as shown inFIG. 7. On the other hand, in the case where the copper oxide layer 14is formed in the above-mentioned oxidation treatment process, anothercopper oxide layer is not newly formed since the surface of the basematerial 21 is already covered by the copper oxide layer 14.

Here, in the case of FIG. 7A where the copper of the exposed basematerial 21 is oxidized thermally and the copper oxide layer 22 isformed, the additive elements in the base material 21 separate andcondense between the copper oxide layer 22 and the base material 21,thereby forming a condensation layer 23. This condensation layer 23corresponds to the above-mentioned brittle layer. On the other hand, inthe case shown in FIG. 7A where the copper oxide layer 14 is formed, acopper oxide layer due to thermal oxidation is not formed, and, thus,the condensation layer 23 is not formed.

After the wire-bonding process, the semiconductor chip 2 is encapsulatedby the seal resin 8. The semiconductor chip 2 is mounted and fixed ontothe die stage 11 of the lead frame 10, and the die stage 11 is alsoencapsulated together with seal resin 8. Therefore, in the process ofFIG. 7A, the copper oxide layer 22 is covered by the seal resin 8 asshown in FIG. 7A-(d). On the other hand, in the process of FIG. 7B, thecopper oxide layer 14, which is forcibly formed by the oxidation processis covered by the seal resin 8 as shown in FIG. 7B-(d).

The formation of the semiconductor device is completed after the resinencapsulation is completed. At this time, the semiconductor devicefunctions normally both in the case of FIG. 7A and the case of FIG. 7B.Accordingly, the semiconductor device is stored until it is used. Duringthe time period of storage, the seal resin of the semiconductor devicemay absorb moisture from a surrounding atmosphere.

Then, when a product is manufactured using the semiconductor device, thesemiconductor device is mounted onto a mounting substrate etc. In manycases, solder mounting is used for mounting the semiconductor device.Especially, a lead terminal type semiconductor device is mounted bysoldering the outer leads to the electrode pads of the mountingsubstrate. In such a mounting process, the semiconductor device issubjected to the heat of the solder reflow. Since a lead-free solder hasa high-melting point, the heating temperature reaches about 230-240° C.

When the semiconductor device is heated at such a temperature, a thermalstress generated in the semiconductor device (seal resin) is increased,which may cause a small crack formed in the brittle condensation layer23. If the moisture, which the seal resin absorbed, enters such a crackand turns into steam exfoliation may occur in the condensation layer 23,as shown in FIG. 7A-(e), and a problem arises in that the seal resin isswollen or broken.

On the other hand, in the case of FIG. 7B where the semiconductor deviceis provided with the copper oxide layer 14 so as to prevent theformation of the condensation layer 23, there is no crack or breakageoccurs near the boundary between the lead frame 10 and the seal resin 8since there is no brittle layer, and, thus, there is no problem such aspackage swelling or cracking.

The inventors produced the base material 21 having the copper oxidelayer 14 according to the present embodiment (in the case of FIG. 7B)and also produced the base material 21 having the copper oxide layer 22which is formed by thermal oxidation (in the case of FIG. 7A), andperformed tape pealing tests on both the copper oxide layers 14 and 22.The lead frames were placed on a heater block heated at 250° C. for 3minutes, and then the copper oxide layers 14 and 22 are attached totapes and pealed from the read frames. As a result, the copper oxidefilm 22, which is a thermal oxidation film, is exfoliated from the basematerial 21 in all of five test pieces. On the other hand, exfoliationdid not occur in the copper oxide layer 14 according to the presentembodiment in all of five test pieces. Therefore, it was proved that thecopper oxide layer 14 according to the present embodiment is joined tothe base material 21 of the lead frame more firmly than the copper oxidelayer 22 formed by thermal oxidation.

As mentioned above, by using the lead frame 10 in which the copper oxidelayer 14 according to the present embodiment is formed, package swellingor cracking due to heating of the semiconductor device in a mountingprocess can be prevented. Especially, even when a solder reflow isperformed at 230-240° C. as in a mounting process using a lead-freesolder, the semiconductor device can be prevented from being swollen orcracked.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority application No.2002-166898 filed Jun. 7, 2002, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. A manufacturing method of a lead frame,comprising: configuring a base material made of a copper alloy into afirst pattern; applying plating to a part of the base material; andforming a copper oxide layer as an outermost layer on a surface of thebase material by immersing the base material into a solution of a strongoxidizer, the copper oxide layer including a copper oxide other than acopper oxide in the form of needle crystals and thickness of the copperoxide layer is from 10 Angstroms to 1000 Angstroms, and taking the basematerial out of the solution of the strong oxidizer before the copperoxide changes to a needle crystal state.
 2. The manufacturing method asclaimed in claim 1, wherein the solution of the strong oxidizer is amixed solution of sodium chlorite, sodium hydroxide and potassiumperoxydisulfate.
 3. The manufacturing method as claimed in claim 2,wherein the immersing the base material into the solution of the strongoxidizer continues for 3 to 10 minutes.
 4. The manufacturing method asclaimed in claim 3, wherein the immersing the base material into thesolution of the strong oxidizer is performed at a temperature of around100° C.